With magnetic recording media having insufficient lubricity, the surface of a magnetic layer is smoothed by contact with contacting parts of magnetic recording/reproducing decks, resulting in an increase in the coefficient of friction of the magnetic layer. When magnetic recording media with such a magnetic layer having an increased coefficient of friction are passed through magnetic recording/reproducing decks, in particular, under high humidity conditions (relative humidity: at least 60%), cohesion occurs between the magnetic recording media and the contacting parts of the magnetic recording/reproducing decks due to the presence of fine water droplets. When the cohesion phenomenon occurs, if the running tension is greater than the cohesive strength, stick-slip running is caused, producing running noises comprising an audible sound. In recording of magnetic recording media under the stick-slip running, signals recorded are modulated in frequency in the running direction, and normal reproduction becomes difficult. When the stick-slip running occurs during the reproduction of recorded signals, phenomena such as wow, flutter and jitter are caused, and normal reproduction cannot be achieved. On the other hand, when the cohesion phenomenon occurs, if the cohesive strength is greater than the running tension, running is stopped and reproduction becomes impossible. These phenomena are called "tape squeal" in the art, and magnetic recording media and magnetic recording/reproducing decks causing such tape squeal are seriously low in product value.
In the case of magnetic recording/reproducing decks, the tape squeal is caused by the material, running tension, running speed, etc. of the magnetic recording medium running system. In the magnetic recording media, smoothing of the surface, an increase in the coefficient of friction, etc. are responsible for tape squeal.
A number of lubricants, solid or liquid, are used to improve the surface lubricity of magnetic recording media, as described in U.S. Pat. Nos. 3,470,021, 3,525,694, 3,630,772, 3,387,993, 3,542,589, 4,018,968, 3,476,596, 3,547,693, 3,471,415, 2,654,681, 3,781,206, 3,625,760, 3,634,253, 3,274,111, 3,492,235, 4,007,314 and 3,476,596. Examples of such lubricants include inorganic or organic fine powders (e.g., Al.sub.2 O.sub.3, graphite, silica, Cr.sub.2 O.sub.3, ZnO, and carbon black) and organic surface active agents (e.g., higher hydrocarbon compounds, aliphatic alcohols, aliphatic acids, aliphatic acid esters, aliphatic acid amides, aliphatic acid salts, and aliphatic acid quaternary salts). There is no limitation on the number of carbon atoms contained in the foregoing compounds, of which are preferred those compounds having a boiling point of at least about 100.degree. C. and a melting point of not higher than about 150.degree. C. The amount of the lubricant added is from 0.1 to 20 parts by weight per 100 parts by weight of the binder.
Addition of such additives does not always succeed in producing a magnetic recording layer having desirable characteristics. For example, the use of large amounts of additives may decrease the mechanical strength of the resulting recording layer and may give rise to the phenomenon that after the formation of the recording layer the additives gradually come out to the surface of the recording layer. Furthermore, dispersion of magnetic substances is not always satisfactory.
For example, the addition of glycerol tristearate as a lubricant to the magnetic layer in an effective amount to inhibit the tape squeal exerts adverse influences on electromagnetic properties, lowering the S/N ratio. When butyl palmitate, which is another example of lubricants, is added to the magnetic recording layer in an effective amount to improve the running properties on guide poles made of stainless steel in the deck running system for magnetic materials, the tape squeal is caused at the deck head portion.